The present invention relates to an optical pickup apparatus which can record information on and read information from a digital video disk (DVD) and a recordable compact disk (CD-R), respectively.
Recording media for recording and reading the information such as video, audio or data, are a disk, a card, or a tape. Among them, the disk type is chiefly used. Recently, in the field of the optical disk apparatus, a laser disk (LD), a compact disk (CD) and a digital video disk (DVD) have been developed. Such an optical disk includes a plastic or glass medium having a certain thickness along an axial direction to which light is incident, and a signal recording surface on which information is recorded and located on the plastic or glass medium.
So far, a high-density optical disk system enlarges a numerical aperture of an objective lens in order to increase a recording density, and uses a short wavelength light source of 635 nm or 650 nm. Accordingly, the high-density optical disk system can record or read signals on or from a digital video disk, and can also read signals from a CD. However, to be compatible with a recent type of a CD, that is, a recordable CD (CD-R), light having a wavelength of 780 nm should be used, due to the recording characteristic of the CD-R recording medium. As a result, using the light beam wavelengths of 780 nm and 650 nm in a single optical pickup becomes very important for compatibility of the DVD and the CD-R. A conventional optical pickup which is compatible with the DVD and the CD-R will be described below with reference to FIG. 1.
FIG. 1 shows an optical pickup using two laser diodes as light sources for a DVD and a CD-R and a single objective lens. The FIG. 1 optical pickup uses laser light having a wavelength of 635 nm when reproducing a DVD, and uses laser light having a wavelength of 780 nm when recording and reproducing a CD-R. Light having the 635 nm wavelength emitted from a laser diode light source 1 passes through a collimating lens 2 and a polarization beam splitter 3 and then goes to an interference filter type prism 4. Light having the 780 nm wavelength emitted from a laser diode light source 11 passes through a collimating lens 12, a beam splitter 13 and a converging lens 14 and then goes to the prism 4, which converges the light having the 780 nm wavelength. An optical system having such a structure is called a xe2x80x9cfinite optical system.xe2x80x9d The prism 4 transmits the light beam having a wavelength of 635 nm reflected from the polarization beam splitter 3, and reflects the light beam converged by the converging lens 14. As a result, the light beam from the light source 1 is incident to a quarter-wave plate 5 in the form of a parallel beam by the collimating lens 2, while the light from the light source 11 is incident to the quarter-wave plate 5 in the form of a divergent beam by the convergent lens 14 and the prism 4. The light transmitting through the quarter-wave plate 5 is incident to an objective lens 7.
The light of the 635 nm wavelength emitted from the light source 1 is focussed by an objective lens 7 on a signal recording surface in a DVD 8 having a thickness of 0.6 mm. Therefore, the light reflected from the signal recording surface of the DVD 8 contains information recorded on the signal recording surface. The reflected light transmits through the polarization beam splitter 3, and is then incident to a light detector 10 for detecting optical information.
If the finite optical system described above is not used, when the 780 nm wavelength light emitted from the light source 11 is focussed on a signal recording surface in the CD-R 9 having 1.2 mm thickness using the above-described objective lens 7, spherical aberration is generated due to a difference in thickness between the DVD 8 and the CD-R 9. In more detail, the spherical aberration is due to a fact that the distance between the signal recording surface of the CD-R 9 and the objective lens 7 is farther than that between the signal recording surface of the DVD 8 and the objective lens 7, along an optical axis. To reduce such a spherical aberration, a construction of a finite optical system including a convergent lens 14 is required. By using a variable aperture 6 to be described later with reference to FIG. 2, the 780 nm wavelength light forms an optimized beam spot on the signal recording surface of the CD-R 9. The 780 nm wavelength light reflected from the CD-R 9 is reflected by the prism 4 and then the beam splitter 13, so as to be detected in the light detector 15.
The variable aperture 6 of FIG. 1 has a thin film structure as shown in FIG. 2 which can selectively transmit the rays of the light incident to the region if not more than the numerical aperture (NA) of 0.6 which coincides with the diameter of the objective lens 7. That is, the variable aperture 6 is partitioned into two regions based on the NA of 0.45 with respect to an optical axis. Among the two regions, a first region 1 transmits both 635 nm and 780 nm wavelength light and a second region 2 totally transmits the 635 nm wavelength light and totally reflects the 780 nm wavelength light. The region 1 has the numerical aperture of 0.45 or below, and the region 2 is an outer region of the region 1 and is made by coating a dielectric thin film. The region 1 is comprised of a quartz (SiO2) thin film in order to remove any optical aberration generated by the dielectric thin film coated region 2. By using the variable aperture 6, the 780 nm wavelength light transmitting the region 1 having the 0.45 NA or below forms a beam spot appropriate to the CD-R 9 on the signal recording surface thereof. Thus, the FIG. 1 optical pickup uses an optimum light spot when a disk mode is changed from the DVD 8 to the CD-R 9. Accordingly, the FIG. 1 optical pickup is compatible for use with the CD-R.
However, the FIG. 1 optical pickup as described above should form a xe2x80x9cfinite optical systemxe2x80x9d with respect to the 780 nm wavelength light in order to remove any spherical aberration generated when changing a DVD compatibly with a CD-R. Also, due to the optical thin film, that is, the dielectric thin film, which is formed in the region 2 having the NA of 0.45 or above, an optical path difference between the light transmitting the region 1 having the NA of 0.45 or below and that transmitting the region 2 having the NA of 0.45 or above, is generated. To eradicate this difference, it is necessary to form an optical thin film in the region 1. Due to this reason, a quartz coating is formed in the region 1 and a multi-layer thin film is formed in the region 2. However, such a fabricating process does not become only complicated but also adjustment of the thickness of the thin film should be performed precisely in units of xe2x80x9cxcexcm.xe2x80x9d Thus, it has been difficult in mass-producing the optical pickup.
An object of the present invention is to provide an optical pickup apparatus which is compatible with a digital video disk and a recordable compact disk by removing a spherical aberration using a phase plate.
To accomplish the above object of the present invention, there is provided an optical pickup apparatus for at least two optical recording media, which are different in distance from an optical pickup to information recording surfaces and uses light beams of different wavelengths for recoding and reading information, the optical pickup apparatus comprising:
a plurality of laser light sources for emitting a first light beam having a relatively shorter wavelength and a second light beam having a relatively longer wavelength, respectively;
an objective lens having a predetermined focal length in which the focal point of the objective lens according to the first light beam coincides with the position of the information recording surface in a first optical recording medium having the information recording surface closer to the objective lens;
optical detection means;
means for controlling an optical path so that the light beam emitted from the laser light sources is directed to the objective lens and the light output from the objective lens is directed to the optical detection means; and
phase shift means, coupled between the optical path control means and the objective lens, for shifting the phase of the second light beam proceeding from the optical path control means to the objective lens, thereby reducing the size of a beam spot which is formed on the position of information recording surface in the second optical recording medium having the information recording surface farther from the objective lens by the second light beam focussed by the objective lens,
wherein a wavelength of one of the first light and second light beams is used according to the optical recording medium used.